In radio transmitters for broadcast, cellular and satellite systems the power amplifier in the transmitter has to be very linear in addition to being able to simultaneously amplify many radio channels (i.e. frequencies) spread across a wide bandwidth. High linearity is required since nonlinear amplifiers would cause leakage of interfering signal energy between channels and distortion within each channel.
In radio transmitter stations for cellular systems, amplifiers in class A and B have been suggested for use in combination with LINC (LINC, Linear Amplification using Nonlinear Components) or Chireix outphasing methods providing high linearity and efficiency, and wide bandwidth.
The outphasing method, which will be described in more detail in FIG. 1, resolves an envelope-modulated bandpass waveform sin in a signal component separator into two out-phased constant envelope signals s1 and s2, which are applied to power amplifiers. The outputs of the power amplifiers are combined in a hybrid arrangement recovering the envelope-modulated waveform. The output amplitude of the amplified outsignal sout is a result of the phase shift between the signals s1 and s2. When the signals are in phase amplitude maximum is achieved and when in anti-phase a minimum amplitude is achieved. The hybrid is constructed in order for the amplifier to see an impedance load. Hence, the efficiency ratio will be reciprocally proportional to the ratio between peak power and mean power. By replacing the impedance load by a compensating reactance network, known as the Chireix method, the region of high efficiency is extended to include lower output power levels.
LINC and Chireix networks are sensitive systems that fit well with unbalanced amplifier like unbalanced class A, B, C, E and F amplifiers.
Documents WO2004/023647 and WO2004/057755 describe composite amplifier structures comprising several Chireix pairs of unbalanced power amplifiers for use in radio terminals such as mobile radio terminals and base stations.
A current-mode class-D power amplifier achieving high efficiency at radio frequencies is described in “Current-Mode Class-D Power Amplifiers for High-Efficiency RF Applications”, IEEE Transactions on Microwave Theory and Techniques, vol. 49, no 12, December 2001, pp. 2480-2485. However, due to new progress in semiconductor electronics, especially production methods of integrated circuits in Gallium Nitride (GaN) techniques, it has been interesting to use balanced class B, E/F and current mode class D (CMCD) amplifiers instead. Said amplifiers are non-linear, but provide high efficiency.
No specific solution exists as to combine balanced amplifiers like the CMCD or class E/F with a LINC network. The main problem is that the CMCD amplifier and class E/F have a balanced output and the LINC network is single-ended so that a high power low loss transformer that works at several impedance levels is needed, which is hard to realize at cellular frequencies.